A conventional thin-film light-emitting diode is shown and described for example in the European patent application EP-A-0 905 797. The thin-film principle utilized in this case is based on internal multiple reflections, connected with an internal scattering of the light beams. In this case, the designation “thin” relates to the optical thickness of the light-emitting diode, that is to say is to be understood in the sense of “optically thin”. Between two scattering reflections, the absorption incurred by a light beam is intended to be as low as possible.
A thin-film light-emitting diode chip is distinguished in particular by the following characteristic features:                a reflective layer is applied or formed at a first main area—facing toward a carrier element—of a radiation-generating epitaxial layer sequence, which reflective layer reflects at least part of the electromagnetic radiation generated in the epitaxial layer sequence back into the latter;        the epitaxial layer sequence has a thickness in the region of 20 μm or less, in particular in the region of 10 μm; and        the epitaxial layer sequence contains at least one semiconductor layer having at least one area which has an intermixing structure which ideally leads to an approximately ergodic distribution of the light in the epitaxial layer sequence, i.e. it has an as far as possible ergodically stochastic scattering behavior.        
A basic principle of a thin-film light-emitting diode chip is described for example in I. Schnitzer et al., Appl. Phys. Lett. 63 (16), Oct. 18, 1993, 2174-2176, the disclosure content of which is in this respect hereby incorporated by reference.
The external efficiency of a thin-film light-emitting diode can be reduced in particular by the active layer of the light-emitting diode itself having a high absorption for the emitted radiation. This is the case for example with AlGaInP/GaAs-based light-emitting diodes in the yellow spectral region. It is often necessary, for reasons other than those associated with the thin-film principle, for instance in order to increase the internal efficiency, the temperature stability or the like, for the layer thickness of the active layer to be chosen to be sufficiently large. This results in that the active layer itself has an appreciable absorption. By way of example, in the case of a yellow AlGaInP/GaAs thin-film light-emitting diode, it may be necessary for the layer thickness to be chosen to be that large, that the absorption for passage of a light beam becomes greater than 10%.
On account of the comparatively low maximum barrier heights for electrons, the internal efficiency of a yellow-emitting active layer comprising the AlGaInP material system depends greatly on the charge carrier density in the active layer and thus on the layer thickness. FIG. 3 shows an empirical profile 70 of the internal efficiency Eint of a yellow AlGaInP active layer as a function of the layer thickness d.
The coupling-out efficiency Eout for such layers, that is to say the ratio of the number of coupled-out photons to the number of photons emitted in the semiconductor crystal, is illustrated in FIG. 4 likewise as a function of the thickness of the active layer d (curve 72). The values shown originate from a ray tracing simulation.
With these two quantities, the utilizable external efficiency Eext results from multiplication of the coupling-out efficiency and the internal efficiency,Eext=Eout*Eint.
The resulting dependence of the external efficiency Eext on the layer thickness d is illustrated for a conventional yellow thin-film light-emitting diode in FIG. 5 by the curve 74. Since the internal efficiency Eint rises sublinearly with the layer thickness, and the coupling-out efficiency Eout falls approximately linearly with the layer thickness in the region of interest, the external efficiency Eext has a maximum which, in the example shown, lies at a thickness of the active layer of about 300 nm. The external efficiency Eext that can maximally be achieved at this layer thickness lies at a relatively low level of about 0.05. This approximately corresponds to what can likewise be achieved with a customary AlGaInP light-emitting diode, not operating according to the thin-film principle, in the yellow spectral region.